The present invention relates to a method of exchanging chlorine atoms for fluorine atoms in ring-fluorinated pyridine compounds.
(Trifluoromethyl)pyridine compounds are commercially valuable chemical intermediates useful in the preparation of medicinal agents and agricultural chemicals. (Trifluoromethyl)pyridine compounds are generally prepared by fluorinating a (trichloromethyl)pyridine compound.
Problems associated with the preparation of (trifluoromethyl)pyridine compounds include (1) an over fluorinated end product, i.e., ring-fluorinated, (trifluoromethyl)pyridines and (2) the formation of a ring-fluorinated isomer of the desired (trifluoromethyl)pryidine product generally described as a fluoro-(chlorodifluoromethyl)pyridine compound. These ring-fluorinated by-products reduce the yield of the desired (trifluoromethyl)pyridines and necessitate additional separatory procedures which are both bothersome and expensive. The ring-fluorinated isomer is a particularly annoying by-product because of the difficulty in separating it from the desired (trifluoromethyl)pyridine product.
EPO Application No. 80201077.7 (Publication number: 0 028,870) teaches the preparation of 2-chloro-5-(trifluoromethyl)pyridine compounds employed as intermediates in the preparation of 2-pyridinyloxy(or thio)phenoxy alkanoic acids and derivatives thereof which are herbicides. It discloses that the formation of the 2-fluoro pyridine is of no practical disadvantage since the halogen at the 2-position is displaced in the subsequent reaction with the metal salt of the hydroxy phenoxy alkanoic acid compound. However, having a fluoro in the 2-pyridine ring position poses a waste stream problem with a metal fluoride (KF, NaF) when compared to a waste stream of NaCl or KCl when a chloro is in the 2-pyridine ring position. Furthermore, converting the 2-fluoro pyridine compounds to 2-chloro pyridine compounds according to the present invention allows for the recovery of HF which can be recycled for use as a fluorinating agent. It is clearly evident that there is a need for a method of converting the 2-fluoro-pyridine by-products to the desired 2-chloro-pyridine compounds.
Heretofore, a method of displacing fluorine atoms from a pyridine ring with a chlorine atom has not been disclosed.